In the area of implantable medical devices, there has been a substantial effort to develop sensors for obtaining information from a body organ such as the heart, or relating to a body function such as respiration. For these purposes, catheters and leads have been widely used with medical devices, both external and implantable, including pacemakers, cardioverter/defibrillators, drug dispensers, cardiac monitors, and a variety of different types of stimulators. The common system arrangement is to have one or more catheters, or leads which interconnect the device with the body organ or body location. The terms catheter and lead are used interchangeably here; as used in this specification, either a lead or catheter connects the device to the body location so as to transmit electrical signals between its distal end and the device, and/or pressure or other signals from the body location to the device. A pacing lead, for example, may include one or more electrodes at about its distal end, and a conductor running the length of the lead to transmit stimulus pulses to the heart and conduct heart signals back to the pacemaker. It is also known to have sensors incorporated into the lead for sensing parameters for operational and diagnostic use, with additional conductors connecting the sensor signals back to the proximal end of the lead/catheter, for connection to the pacemaker or other device. In addition to sensing cardiac electrical activity, sensors are used for sensing, eg, blood pressure waves, acoustic waves, respiratory sounds, etc. Thus, for a wide variety of applications there is a need for efficient transmission of signals from a body location to an implanted device. Although this invention embraces various such applications, it will be illustrated primarily in the environment of the preferred embodiment, a pacemaker system.
Modern pacemaker systems have evolved greatly beyond the initial pacemakers which simply delivered a fixed rate of pacing pulses. Pacemakers are widely programmable to operate in different modes and to operate with different pacing parameters. Specifically, many pacemakers are rate responsive, meaning that they automatically sense the patient's demand, or need for rate variation, and adjust pacing rate accordingly. Pacemaker systems are also incorporating more sensor information relating to the patient's metabolic needs and cardiac history. The ability of the pacemaker to undertake additional diagnostic functions, and to accurately adapt pacemaker performance to metabolic needs, is dependent upon good sensor information.
As is well known, rate responsive or rate adaptive pacemakers may utilize any one of a number of different sensors for obtaining different physiologically based signals. Sensors that provide an indication of actual heart performance are coming into greater use. For example, sensors are used for measuring the pressure inside the patient's right ventricle, intramyocardial pressure, or myocardial contractility. Sensing pressure within the patient's heart is known to offer good potential for accurate determination of the patient's needs. See U.S. Pat. No. 5,353,800, assigned to Medtronic, Inc., which provides a discussion of the many different types of pressure sensors used in cardiac pacing systems.
As discussed in the prior art, the approach to measuring pressure changes within the heart has generally involved special leads adapted to carry a sensor which is located within the heart. Thus, a pressure sensor is located on the pacing lead close to the distal tip end, preferably positioned to maximize the sensor response. Such a lead requires extra wires throughout the length of the lead, for interconnection of the sensor signal to the pacemaker. Further, packaging a sensor in a lead tip, while maintaining the requisite minimal lead dimensions, presents considerable difficulty. Thus, it would be advantageous, both for newly implanted pacing systems and for replacement systems, to provide the pacemaker itself with one or more pressure sensors which receive pressure signals representative of cardiac movement, which signals are transmitted through a standard pacing lead and delivered to the pacemaker-mounted sensor. Such an arrangement, as presented by this invention, renders unnecessary any special lead construction, and by-passes the problems of fabricating a sensor on the lead and properly positioning the sensor within the heart. Further, for a patient requiring pacemaker, or pulse generator replacement, and already having a standard lead, it would clearly be advantageous to be able to replace the pacemaker with one which contains apparatus for reliably receiving a pressure signal transmitted through the implanted pacing lead.
There have been some prior art efforts to provide an implantable system with a catheter or lead which transmits a pressure signal from a body location such as the heart back to the control device, eg, the pacemaker. See, for example, U.S. Pat. Nos. 4,763,646 to Lekholm, and 5,353,800 to Pohndorf et al. These patents provide suggestions of transmitting pressure signals to the interior of a pacemaker can, but do not disclose efficient structure for achieving this. There thus remains a significant need in the implantable device art, and the pacemaker art in particular, for a system which provides for reliable and useful chronic transmission of signals such as pressure signals from an interior body location to the implanted device.